1. Field of the Invention
The present invention relates to a method for detecting an operating mode in a wireless communication system, and more particularly to a method for detecting an initial operating mode in a wireless communication system employing an Orthogonal Frequency Division Multiple Access (OFDMA) scheme.
2. Description of the Related Art
In general, a wireless communication system is a system for supporting wireless communication services, and includes a Base Station (BS) and a Mobile Station (MS). The BS and the MS communicate with each other by using transmission frames. In order to transmit and receive the transmission frames, the BS and the MS must acquire mutual synchronization. In order to acquire the mutual synchronization, the BS transmits a synchronization signal to the MS to enable the MS to synchronize with the start of the frames transmitted from the BS. Then, the MS receives the synchronization signal transmitted from the BS to confirm the frame timing of the BS and decode received frames according to the confirmed frame timing. A specific preamble sequence, which is communicated between the BS and the MS, is usually used as the synchronization signal.
In a wireless communication system employing an Orthogonal Frequency Division Multiplexing (OFDM) scheme or an OFDMA scheme (that is, an OFDM wireless communication system or an OFDMA wireless communication system), a preamble sequence having a small Peak to Average Power Ratio (PAPR) must be used.
The reason why the preamble sequence of the OFDM or the OFDMA wireless communication systems must have a small PAPR is as follows.
First, the OFDM wireless communication system is a multi-carrier communication system in which data is transmitted/received at high speed by using a plurality of sub-carriers, or sub-channels including at least one sub-carrier. Orthogonality between the respective sub-carriers is important. On that account, phases are set such that the sub-carriers have mutual orthogonality. Nevertheless, when the phases are changed during signal transmission/reception over the sub-carriers, signals may overlap between the sub-carriers. In this case, the amplitudes of the signals which overlap due to the phase change become out of synchronization with a linear interval of an amplifier provided in the OFDM wireless communication system, and thus it is impossible to normally transmit/receive the signals. For this reason, the OFDM wireless communication system uses a preamble sequence having a minimum PAPR.
In addition, the OFDM wireless communication system transmits data to many users, that is, many MSs by multiplexing one frame with respect to time. In the OFDM wireless communication system, a frame preamble indicating the start of the frames is also transmitted for a certain period from the starting point of the frames. Furthermore, since data to be transmitted to the respective users may be irregularly transmitted within one frame, a burst preamble indicating the start of the data exists in the front of the respective data. Thus, the MS must receive the data preamble in order to determine the starting point of the data transmission. That is, the MS requires synchronization with respect to the starting point of the data transmission in order to receive the data. To this end, the MS must adjust the synchronization by seizing a preamble sequence, which is used in common in all systems, before receiving the signals.
FIG. 1 is a diagram illustrating a downlink frame structure of a wireless communication system employing an ordinary OFDMA scheme.
Referring to FIG. 1, the down frame includes a preamble section 102, a section 104 consisting of a Frame Control Header (FCH), a DownLink Map (DL-MAP) and an UpLink Map (UL-MAP), and data transmission sections 106, 108, 110, 112. DL-MAP is a term used in IEEE 802.16 networks to define the usage of the downlink intervals for a burst mode PHY (Physical Layer), and UL-MAP is a term used to describe a MAC (Medium Access Control) message that defines burst start times for both time division multiplex and time division multiple access by a subscriber station on the uplink.
A synchronization signal for acquiring mutual synchronization between the BS and the MS, that is, a preamble sequence, is transmitted through the preamble section 102. In the FCH and DL/UL-MAP section 104, the FCH includes a location of the DL/UL-MAP and information on a sub-channel configuration method for data transmission in the subsequent downlink frame periods, a channel coding method and so forth. Therefore, the MS cannot acquire information on subsequently transmitted symbols before decoding the FCH. Also, the DL/UL section includes broadcasting control information.
The data transmission sections 106, 108, 110, 112, which are illustrated by way of example, may be divided into a Partial Usage of Sub-Channels (PUSC) zone 106, a Full Usage of Sub-Channels (FUSC) zone 108, an optional FUSC zone 110, and an Adaptive Modulation and Coding (AMC) zone 112. The respective data transmission sections 106, 108, 110, 112 can be distinguished from each other on the same frame by time division.
A brief discussion about the respective data transmission sections is as follows.
First, the PUSC zone will be described. The PUSC zone is a data burst section in which sub-channels are configured using a PUSC scheme. In other words, the PUSC scheme is a sub-channel configuration scheme in which only partial sub-channels of all of the sub-channels are assigned and used on a sector by sector basis, and the frequency reuse rate is above 1. Thus, by assigning different PUSC sub-channels from each other to sectors of two neighboring cells, mutual interferences between the sectors can be removed.
Secondly, the FUSC zone will be described. The FUSC zone is a data burst section in which sub-channels are configured using a FUSC scheme. In other words, the FUSC scheme is a sub-channel configuration scheme in which all of the sub-channels are assigned to and used in all sectors of all cells, and the frequency reuse rate is 1. In the FUSC scheme, all of the sub-channels may be used in all the sectors, but the sub-carriers constituting the sub-channels are set differently from sector to sector in order to minimize sub-channel interferences between the sectors. That is, the FUSC sub-channels are designed such that hit probabilities between sub-carriers constituting the sub-channels are minimized.
Thirdly, the optional FUSC zone will be described. Similar to the FUSC zone, the optional FUSC zone uses the FUSC scheme, but a mathematical equation for configuring sub-channels is different from that of the FUSC zone. Lastly, the AMC zone will be described. The AMC zone employs a scheme in which the entire frequency band is divided into specific frequency bands, and the specific frequency bands are adaptively assigned to the MSs while different modulation and coding methods are applied according to the divided frequency bands.
Sub-channel coding methods include the following four scheme: a Convolutional Coding (CC) scheme, a Convolutional Turbo Coding (CTC) scheme, a Block Turbo Coding (BTC) scheme and a Zero Tail Convolutional Coding (ZTCC) scheme.
As stated above, in order to transmit data, it is required to adjust synchronization between the BS and MS by use of the preamble, and decode the FCH and DL/UL-MAP section. As an example, in the IEEE (Institute of Electrical and Electronics Engineers) 802.16 communication system, it is prescribed that synchronization between a BS and an MS is adjusted first, a FCH and a DL/UL-MAP are decoded, and then a specific one of the above-mentioned operation modes for data transmission is selected for the data transmission. According to the current standards of the IEEE 802.16, it is a prerequisite that the PUSC scheme should be employed as a sub-channelization method for use in initial operation mode determination, and the CC scheme should be used as a sub-channel coding method.
However, placing restriction on selecting the initial operating mode, that is, a limit to using specific schemes as stated above, has acted as an inefficient factor in system design and administration. This is because there may occur a situation where operators or developers cannot use sub-channelization and sub-channel coding methods for initial operation mode determination, which are prescribed as an essential condition for a specific system. In this situation, there is a problem in that the above-mentioned restriction on initial operating mode ultimately causes an unnecessary waste of resources in the current wireless access communication system.